The present invention relates, in general, to semiconductor devices, and more particularly, to a novel low threshold current, high efficiency semiconductor laser.
Semiconductor lasers had long been produced by the semiconductor industry. Of particular interest in recent years had been quantum well heterostructure lasers and especially strained layer quantum well heterostructure lasers. Strained layer structures referred to combining materials that have large differences in their lattice constant, that is, spacing of the atoms in the material's lattice structure. Indium arsenide and gallium arsenide represent an example of two materials that had large differences in their lattice constants. When such materials were combined to produce a semiconductor laser, the difference in the lattice constants developed strains or stress between the materials which limited the thickness of the materials. If the materials' thicknesses were not limited, the strain could cause misfit dislocations or cracks that resulted from the misfit of the two materials' lattice structures. Such cracks or misfit dislocations could result in failure of the semiconductor device. The potential of misfit dislocations limited not only the thickness of materials that had dissimilar lattice constants, but it also limited the ratio or mole fraction of materials that could be combined in a compound. Use of these materials in a heterostructure semiconductor resulted in limitations on carrier concentrations and location of the carriers due to the material's composition restrictions. Since the threshold current and the efficiency of a strained layer quantum well heterostructure laser were determined by carrier concentrations and placements in the laser's structure, the material composition limitations restricted the laser's threshold current and its efficiency. Even though the typical threshold current of such lasers was approximately one milliamp., it was larger than desired. Additionally, the value of the threshold voltage required to develop the threshold current was higher than desired.
Accordingly, it is desirable to have a strained layer quantum well laser that has a low threshold current (less than one milliamp.), that has a low threshold voltage, and that has a high efficiency.